Biochemical and pharmaceutical applications have requirements for rapid screening and detection of compounds in extremely complex mixtures. Advances in chemical analysis technology applied to these fields must achieve a high degree of specificity in separations and incorporate systems that avoid slow separations, especially those involving chromatography and electrophoresis.
At present, the compounds in complex mixtures are separated and analyzed by chromatographic and electrophoretic methods combined with atmospheric pressure ionization-mass spectrometry (API-MS). In these separation techniques, a portion of a sample is introduced as a discrete pulse into the sample inlet of the API-MS system. The sample components are separated either through a component-specific interaction with mobile or stationary phases, or by differences in the drift velocities of components under the influence of electric fields. Because of the time that it takes for the components to migrate, chromatographic and electrophoretic methods require relatively long time periods to accomplish the separation, on the order of several minutes, whereas analysis by mass spectrometric methods provides data almost immediately. In practice, therefore, the mass spectrometer spends significant periods of time waiting for the arrival of transient signals. This is inefficient since the separation technology is very much less expensive than the MS instrumentation.
The above-mentioned problem is reduced when the separation technology operates in a continuous mode, for example the mixture is continuously delivered to the inlet of the separator and the selection of the separated components is electronically controlled. In this manner the MS acquires measurements of selected components in the mixture at almost full efficiency. Optionally, the MS is used to continuously study a particular component in a mixture until sufficient information is acquired. As will be obvious to one of ordinary skill in the art, operation of the separation technology in a continuous mode is impossible using existing chromatographic and electrophoretic techniques because the component of interest arrives only as a transient at the end of the separation. This transient mode of operation limits significantly the number and types of experiments that can be conducted during the lifetime of a given transient signal. Furthermore, if the information that is acquired during the transient is insufficient, a new sample must be injected and a delay is encountered during which the components are being separated.
Alternatively, complex mixtures may be studied using tandem mass spectrometry (MS/MS). With this technology, the ions are selected by a first mass analyzer operating at low pressure (e.g., 1×10−5 torr) inside the vacuum chamber of a mass spectrometer, and are directed to enter a gas cell which is held at a higher bath gas pressure (e.g., 1×10−3 torr). Upon entering this chamber, the ions collide with the molecules of bath gas and, if the kinetic energy of the ion is sufficient, the ion dissociates into some compound-specific fragments. The fragments pass out of the higher-pressure gas cell and are analyzed using a second mass analyzer, operating at a lower pressure, similar to that of the first mass analyzer. The advantage of tandem mass spectrometry is that the specificity is exceedingly high because of compound-specific fragmentation patterns that are created during the collision-induced dissociation. However, tandem MS requires considerable method development time and the operator must have expertise to operate the instrument. Furthermore, tandem MS cannot effectively quantify many kinds of isomeric ions (e.g., leucine and isoleucine) when both components coexist in the mixture. Accordingly, tandem MS is most suited to applications based on target compound analysis, where the system is used to search for a series of expected compounds and the identity of the expected fragment ions is known. Under these conditions the MS/MS experiment is capable of detecting ions at exceedingly low abundance, even in the presence of interfering compounds, since the MS/MS spectrum is very compound-specific. Tandem MS is less effective when used to study mixtures containing unknown components at trace concentrations. Since the existence of these unknowns cannot be predicted, the mass spectrum of the mixture must have peaks which are discernible above the background noise. In particular, detection of low intensity ions is a problem when using the electrospray ionization (ESI) technique, since ESI produces background ions that elevate the baseline intensity along the mass-to-charge ratio axis of a mass spectrum. This background of ions makes detection of unknown trace components difficult, if not impossible.
Of course, complex mixtures may also be analyzed using mass spectrometers with extremely high resolution, such as FT-ICR systems. However, high resolution mass spectrometers are very expensive.
FAIMS is a relatively new separation technique, which solves a number of the problems that are associated with the above-mentioned prior art techniques. FAIMS separates ions on a continuous basis, with the separation occurring under electronic control. Additionally, FAIMS reduces the background chemical noise inherent to atmospheric pressure ionization techniques, thus reducing the detection limits for unknown components in complex mixtures. Finally, FAIMS optionally is operated in tandem with many of the other technologies that are noted above, because the FAIMS device is located between the ion source and the mass spectrometer. A consequence of this physical location is that the FAIMS apparatus can be operated in conjunction with chromatography, electrophoresis, tandem mass spectrometry and high resolution mass spectrometry, etc.
Typically, ions are introduced into a FAIMS device after being formed by atmospheric pressure ionization, such as for instance corona discharge ionization, ionization by radioactive Ni, and electrospray ionization as just a few non-limiting examples. In each of these cases, the sample is one of a liquid and a gas, and in every case the analyte ions are suspended in a gas. One notable exception is found in U.S. Pat. No. 6,653,627, issued on Nov. 25, 2003 in the name of Guevremont et al., which discloses a FAIMS apparatus and method using a laser based ionization source. The entire contents of U.S. Pat. No. 6,653,627 are incorporated herein by reference. In that case, a matrix-supported sample is deposited on a target surface that is disposed within the FAIMS analyzer region, and irradiation is performed using a laser that is disposed external to the FAIMS analyzer region. Since ions are formed within the analyzer region, problems associated with low ion transmission efficiency through an ion inlet are eliminated. Unfortunately, in order to introduce new sample it is necessary to disassemble the FAIMS electrode assembly, remove the existing target surfaces, prepare new target surfaces, introduce the new target surfaces, and finally reassemble the FAIMS electrode assembly. Of course, this sample introduction technique does not support rapid screening of samples, and is very time consuming.
Placing the target surface of the laser source at a location that is external to the FAIMS analyzer would reduce the time and labor that is required for introducing new samples into the FAIMS. In order to achieve high ion transmission efficiency into the FAIMS analyzer region, the target surface should be located as close as possible to the ion inlet orifice of the FAIMS, and should also be disposed parallel to the ion inlet orifice. Unfortunately, when the target surface is disposed for achieving high ion transmission efficiency, very little space remains for arranging the laser light source at a position for irradiating the target surface.
It would be advantageous to provide a method and an apparatus for introducing ions, that are formed using a laser source, through an inlet into a FAIMS analyzer region, with high ion transmission efficiency. It would be further advantageous to provide a method and an apparatus for introducing such ions in a manner that supports rapid screening of samples.